
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12998-2
10.1016/j.heliyon.2024.e36967
e36967
Review Article
The research progress of rare earth agricultural light conversion film
Wang Jing
Qiao Xin
Li Bo
Liu Bo
Zhang Juan
Yan Zhen
Hao Pengcheng
Wang Xuanhang
Liu Yayuan
Shen Leijun wangshenleijun@126.com
⁎
Wang Zhongzhi 198322wzz@brire.com
⁎⁎
Baotou Research Institute of Rare Earths, Baotou, 014030, China
⁎ Corresponding author. wangshenleijun@126.com
⁎⁎ Corresponding author. 198322wzz@brire.com
27 8 2024
15 9 2024
27 8 2024
10 17 e3696723 1 2024
5 8 2024
26 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
The light-converting agricultural film is a new high-end functional film that can convert ultraviolet light and yellow-green light, which is harmful or useless to plant photosynthesis, into red-orange light or blue-violet light required for photosynthesis. The potential advantages of light-converting agricultural film in efficiently utilizing solar energy and improving crop yield have attracted more and more attention from researchers and agricultural enterprises.The light-converting function is realized by adding a light-converting agent to the agricultural film. Therefore, the preparation of light-converting agents with excellent performance is the core of the development and utilization of light-converting agricultural films. The paper firstly summarizes the key research and development in the field of agricultural light-converting films in china. Then this paper summarizes the classification of light-converting agents, research progress, and preparation methods. Finally, this paper predicts the future development trend of light-converting agricultural films, in order to provide a reference for the research and development of stable and efficient light-converting agricultural films.

Keywords

Light-converting agricultural film
Light-converting agent
Light-converting masterbatch
Rare earth
==== Body
pmcWith the gradual expansion of population base and the continuous upgrading of residents' consumption structure, the carrying capacity of land resources and environment is becoming increasingly tense, and China's food production and demand will still maintain a tight balance [1]. Improving the unit yield of crops can not only effectively solve the problem of food and clothing for the population, but also benefit the rapid development of China's modern economic construction. Therefore, it is imperative to efficiently develop ecological agricultural technology.

There are many main factors that determine crop yield and quality, including seed quality, different chemical fertilizer choices, different cultivation methods, and the light required for plant photosynthesis. Compared with external factors such as chemical fertilizers and cultivation methods, how to effectively utilize sunlight in nature for photosynthesis is a simpler, cheaper, safer, more environmentally friendly, and more convenient and practical way to improve crop yield and quality [[1], [2], [3]]. The growth and development of crops require the participation of sunlight, and different wavelengths of sunlight have different effects on plants. The researches has shown that [2,4,5], the blue-purple light with a wavelength range of 400–480 nm can increase the protein content in crops and effectively promote the growth of plant stems and leaves; the red-orange light with a wavelength range of 600–700 nm can increase the formation of carbon hydrates and promote the normal growth and reproduction of plant fruits; however, the yellow-green light with a wavelength range of 500–600 nm has almost no contribution to the process of plant photosynthesis; in addition, the near-ultraviolet radiation with a wavelength range of 280–390 nm not only increases plant diseases and pests, but also significantly accelerates the aging of plastic agricultural film structures. Therefore, how to effectively convert the yellow-green light and ultraviolet light that are not beneficial or harmful to crop growth in sunlight into blue-purple light and red-orange light that are conducive to crop photosynthesis has become a key issue [1].

This article will start with the key research and development in the field of light conversion agricultural film in China, and provide an overview of the classification of light conversion agent, preparation methods of light conversion agent, research status of light conversion agent, and future development prospects of light conversion agricultural film industry in China.

1 Light-converting agricultural film

In recent years, with the increasing development of new agricultural science and technology, light conversion agricultural film has gradually become a new trend in the field of agricultural technology research and an important direction for the development of new generation agricultural film functionalization technology [[6], [7], [8], [9], [10], [11]]. The light conversion agricultural film is a new high-end functional film that can convert harmful ultraviolet light or useless yellow-green light in sunlight into red-orange light and blue-purple light, which is beneficial for plant photosynthesis [12,13]. Fig. 1 is a schematic diagram of the light conversion process of light conversion agricultural film.Fig. 1 The schematic diagram of the light conversion process of the light-converting agricultural film.

Fig. 1

In recent years, with the rapid development of agricultural technology and the widespread use of agricultural films, the country has also put forward higher requirements for the use of agricultural films. In the future, the overall development of China's agricultural film industry will focus on the innovation, research and development and utilization of high-end agricultural films (light conversion agricultural films). It is expected that by 2026, the production of agricultural film in China may reach around 1.05 million tons [12], which is of strategic significance for the production of agricultural film and the research and development of high-end agricultural film (light conversion agricultural film).

China has been conducting systematic research on agricultural light conversion films since the late 1980s. In 1985, china imported agricultural light conversion blue film from Japan and applied it to the cultivation of rice seedlings, achieving significant high yield planting benefits. This is mainly due to the favorable growth of rice seedlings caused by the blue light in sunlight, which not only increases the incident flux of sunlight, but also inhibits the proliferation of pathogenic bacteria in crops. However, due to its high price, it cannot be promoted in the field of agricultural production [14].

In 1991, the plastic factory in Yichun City, Heilongjiang Province developed a light conversion agricultural film that can convert ultraviolet light into red and blue light, and verified the yield increase effect of the light conversion agricultural film on crops such as cucumber and ginseng [15]. In 1993, Fu et al. from Shanghai Normal University successfully added rare earth fluorescence complex to PE (Polyethylene) film. Compared with ordinary membranes, the transmittance of the new membrane has increased by 4 %–18 %, which can increase the yield of tomatoes and eggplants [16]. In 1993, Professor Li et al. from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences [10] added rare earth organic complex light conversion agents to PVC (Polyvinyl chloride) films to make plastic agricultural films for vegetable greenhouses and used them to cultivate rice, ginseng, and vegetables, achieving a certain yield increase effect. In 1994, Beijing Agricultural University, Institute of Chemistry of the Chinese Academy of Sciences et al. prepared the "Ruidelai” light conversion masterbatch by learning from the light conversion technology of Russia, and then cooperated with other companies to make light conversion agricultural films from light conversion masterbatch to carry out large-scale shed fastening tests [17]. In 1996, Professor Lian from Hunan Normal University [18] developed the light conversion agricultural red film, which can promote the early maturity of crops and increase crop yield. In 1999, the first plastic factory in Jiaozuo City, Henan ProvinceJiaozuo developed a new type of inorganic rare earth light conversion agent based on sulfide and mixed with Mn2+and Eu2+activators. The new type of inorganic light conversion agent can absorb the ultraviolet light and yellow green light in the sunlight and emit blue violet light and red orange light. At the same time, the polyethylene high light efficiency light conversion agricultural greenhouse film developed has been found to have significant promoting effects on crop growth and reproduction through field greenhouse experiments and production applications [6]. In 2000, Wang et al. from Shanghai Normal University added rare earth light conversion agents and other additives to PE resin, and then prepare a new type of polyethylene red light conversion agricultural film through three-layer coextrusion technology [19]. This agricultural film has a significant promoting effect on improving the quality and yield of crops. In 2007, Wu et al. from Nanjing Normal University [11] developed a rare earth organic complex light conversion agent, and added it to the resin to make a PVC agricultural film. The light conversion film can absorb the ultraviolet light in the sunlight and convert it into blue violet light and red orange light that are conducive to crop photosynthesis. In 2009, Wang et al. from Shihezi University [20] prepared the light conversion film by adding rare earth ion light conversion agents containing samarium to polyethylene resin. Compared with ordinary films, the light conversion agricultural film can effectively absorb ultraviolet light at 264 nm and generate strong red fluorescence at 650 nm. It has good photothermal stability and good adhesion with polyethylene film. In 2016, Zhang et al. from South China Agricultural University [21] prepared the light conversion film by blending the red-emitting Sr2MgAl22O36:Mn4+ phosphor and polyethylene. The light conversion film possesses a strong emission band at 630–730 nm, matching well the absorption spectrum of chlorophyll complexes. In 2017, Li et al. from Shandong Agricultural University [22] found that the type of light transfer agent regulating the light transfer agricultural film has a significant impact on the light environment of solar greenhouse and the growth and development of sweet pepper, as shown in Table 1. In 2018, Lian et al. from Hunan Normal University uniformly [23] dispersed VTR-660 light conversion agent into a resin to prepare a light conversion agricultural film (T3) with a wide spectral band purple to red function. After covering the T3 light conversion agricultural film, the yield per hectare of tomatoes significantly increased by 25.71 % (P < 0.05) compared to covering the traditional added EVA defogging and drip free film (T1). At the same time, the content of vitamin C and lycopene in tomatoes significantly increased by 11.11 % and 33.04 % (P < 0.05), respectively. It can be seen that the light conversion agricultural film can significantly improve the transmittance of red light in sunlight and improve the lighting environment in the greenhouse to achieve the goal of increasing tomato yield and improving fruit quality, as shown in Table 2. In 2020, Wu et al. from South China Agricultural University [24] prepared a Sr2Si5N8:Eu2+-based light-conversion agricultural film. And the effects of the light-conversion film on the biomass and quality of Chinese flowering cabbage was studied. The results show that the cabbage biomass is increased and the quality is improved with the light-conversion film treatment, which might be related to the photosynthesis of red light-promoting plants, as shown in Fig. 2(a)–(f). In 2022, Li et al. from Hebei Agricultural University [25] applied a new type of rare earth light conversion film (RPO) to cucumber in a solar greenhouse. Compared with PO film (CK), it was found that rare earth light conversion agricultural film can significantly increase the yield of cucumber, with an increase of approximately 30.01 %, as shown in Table 3. In 2023, Zhou et al. from Jinan University [26] prepared a kind of organic complex (light conversion agents) doped with a variety of rare earth ions (Sm3+, Eu3+, and Y3+) by using salicylic acid and o-phenanthroline as ligands. And a light conversion agricultural film with the function of converting ultraviolet light to red light was prepared by mixing the synthesized light conversion agent into ethylene-1-hexene copolymer (PO) film by blow molding. In 2023, Zhang et al. from Changchun Sci & Technols University [27] synthesized a down and up double model conversion CaS: Eu2+, Sm3+phosphor through high-temperature solid-phase method, which can convert infrared and violet-blue light into red light. Then CaS: Eu2+, Sm3+phosphor was applied to agricultural film. The translucent agricultural film has a high transparency of over 70 % in the visible light region, and high absorption characteristics in the ultraviolet (UV) and blue regions. It shows good stability, and the photo-aging is only 19 % after 20 days of continuous exposure to strong ultraviolet radiation.Table 1 The effects of different turning red films on plant growth of sweet pepper [22].

Table 1Treatments	Plant height/cm	Stem diameter/cm	Internode length/cm	Blade longitudinal diameter/cm	Blade transverse diameter/cm	
CK	111.87 ± 6.53c	1.72 ± 0.03a	5.50 ± 0.07bc	14.63 ± 0.40b	15.13 ± 0.39bc	
KMN-1	120.67 ± 3.90b	1.62 ± 0.05b	5.63 ± 0.14b	16.10 ± 0.55b	15.87 ± 0.53b	
KMN-2	126.23 ± 2.81a	1.58 ± 0.10bc	6.10 ± 0.09a	15.10 ± 0.49b	17.17 ± 0.41a	
KMN-3	110.33 ± 4.09c	1.51 ± 0.08c	5.42 ± 0.11c	13.93 ± 0.36c	14.97 ± 0.44c	
(Different lowercase letters indicate significant differences between treatments (P < 0.05), the same below).

Table 2 The effect of different sample films on yields and fruit qualities of tomatoes [23].

Table 2Treatments	Average weight of single fruit/g	Number of harvested fruit per plant	Yield (t·hm−2)	vitamin C (mg·100g−1)	Lycopene (ug·g−1)	Total solube solid/%	Nitrite (mg·kg−1)	
T1	201.7 ± 2.08c	25.3 ± 0.58b	163.764 ± 2101.95c	13.5 ± 0.31c	34.2 ± 0.09c	5.97 ± 0.13b	0.37 ± 0.04a	
T2	211.7 ± 2.98b	26.3 ± 0.58b	178.739 ± 3618.45b	14.3 ± 0.47b	40.7 ± 0.17b	6.20 ± 0.15a	0.35 ± 0.05a	
T3	226.6 ± 2.50a	28.3 ± 0.58a	205.874 ± 4717.65a	15.0 ± 0.31a	45.5 ± 0.13a	6.30 ± 0.10a	0.18 ± 0.07b	

Fig. 2 Different parameters of Chinese flowering cabbages obtained under the two films: (a) Biomass, (b) Leaves number, (c) Weight, (d) Photosynthetic pigment content, (e) Soluble protein content and the polyphenol content, (f) Soluble sugar content [24].

Fig. 2

Table 3 The effect of rare earth light conversion film on cucumber fruit yield [25].

Table 3Treatments	Average fruit weight/g	Fruit per plant/pcs	Plot yield (kg·80m−2)	Converted mu amount (kg·667m−2)	
Guava period	Shenggua period	Senescence	
CK	159.04 ± 5.53b	188.98 ± 8.05b	140.21 ± 14.89b	6.33 ± 0.58b	467.63 ± 11.87b	3898.87 ± 11.87b	
RPO	189.19 ± 17.61a	206.61 ± 11.91a	167.43 ± 5.48a	7.69 ± 0.58a	607.95 ± 10.21a	5068.78 ± 10.13a	

At present, China's agricultural light conversion film has achieved certain results in agricultural demonstration applications, but there are also areas where the effect is not significant in certain applications. The main problems lie in the following three aspects [1,6].(1) The emission spectrum band of the light conversion agricultural film is too narrow, and the matching degree between the emission spectrum and the absorption characteristic spectrum of plant photosynthesis is low: At present, most of the light conversion agricultural films use compounds containing trivalent rare earth Eu3+. Under UV excitation, the maximum emission peak is mostly the characteristic sharp peak of Eu3+at a wavelength of 613 nm, which has poor matching with the absorption spectrum of plant photosynthesis.

(2) The attenuation period of light conversion of agricultural film is short:According to reports, the fluorescence emission intensity of domestic light conversion agricultural films can usually only maintain 10–30 % of the initial fluorescence emission intensity after continuous outdoor exposure for one year.

(3) Single red light effect of light conversion agricultural film:Due to the dual light gain effect of plant photosynthesis, the effect of increasing red light alone on promoting plant photosynthesis and growth and development is not significant.

2 Light conversion agent

The efficient light conversion performance of light conversion agricultural films is mainly achieved through light conversion agents. Light conversion agents, with their potential advantages in overcoming daily sunlight limitations, efficiently utilizing solar energy, and improving crop yield, are increasingly receiving attention from scientific research institutions and agricultural enterprises. At the same time, they have become promising candidate materials for the next generation of horticulture and agriculture. At present, from the preparation and market application of light conversion agricultural films in China, the development and innovation of new light conversion agents are still the key to achieving light conversion of agricultural films and improving solar light quality. In recent years, although the emission spectra of sulfide type rare earth inorganic light conversion agents reported and studied have a high matching degree with crop photosynthesis spectra, they have drawbacks such as easy deliquescence of sulfides, poor dispersion in agricultural films, and easy aggregation; Although organic compounds and fluorescent dye conversion agents have high luminescence intensity and good dispersibility in resins, they have drawbacks such as sharp emission peaks, poor overlap between emission spectra and plant absorption spectra (as shown in Fig. 3), poor anti attenuation performance, and high cost.Fig. 3 The absorption spectrum of chlorophyll in plants [6].

Fig. 3

Rare earth elements are known as "industrial vitamins” and have excellent optical properties that cannot be replaced. Due to the rich spectral energy levels of rare earth ions, rare earth compounds have many unique chemical and physical properties, and rare earth luminescence almost covers the entire range of solid-state luminescence. The research of light conversion agents prepared by doping rare earth elements and light conversion agricultural films has gradually been valued by researchers. The rare earth light conversion agents are a type of photoluminescent materials, and there is no unified classification standard for them. Wang et al. [28] classified and summarized light conversion agents based on relevant literature reports both domestically and internationally. According to the properties of materials, they can be divided into: rare earth organic complexes, fluorescent dyes, rare earth inorganic compound; According to the luminescent properties of materials, they can be divided into blue light agents, red light agents, and red blue composite agents; According to the light conversion properties of materials, they can be divided into: ultraviolet to red, ultraviolet to blue, and green to red.

2.1 Organic light conversion agent

Organic light conversion agents can be divided into rare earth organic complex light conversion agents and organic fluorescent dye light conversion agents.

2.1.1 Rare earth organic complex light conversion agents

The rare earth organic complexe light conversion agents are mainly formed by the combination and interaction of rare earth ions and organic ligands through coordinate covalent bond. Organic molecules in rare earth organic complexes act as ligands and generate π - π * absorption when excited by high-frequency light. Then the electrons of the central rare earth ion transfer from the ground state to the excited state through energy transmission. When the electrons return to the ground state, the central rare earth ion emits characteristic fluorescence [26]. It is generally believed that the light conversion of rare earth metal complexes undergoes such a process (Fig. 4): the ligand absorbs ultraviolet light and transitions from the singlet state S0 to the singlet excited state (S1). The lifetime of the singlet excited state is very short, and it quickly transitions through the system to the metastable triplet state (T). Then, the lowest excited triplet state (T1) transfers energy to the vibrational energy levels of rare earth ions. At this time, the ground state electrons of the rare earth ion are excited and transition to the excited state [29,30]. When the electrons return from the excited state to the ground state, they emit characteristic fluorescence of each ion.Fig. 4 The schematic diagram of intramolecular energy transfer process in rare earth complexes [29].

Fig. 4

Lu et al. [31] have successfully developed a polymer europium complex, which is composed of a macromolecular ligand β- A ternary complex composed of diketone compounds and trivalent europium. The macromolecular ligand is polystyrene with an 8-hydroxyquinoline unit at the end group.The ternary organic complex is a bifunctional light conversion material that not only has the characteristic red light emission of Eu3+, but also has the blue light emission of macromolecular organic ligand. It can be stably and evenly dispersed in the polyethylene film. Zhao et al. [9] synthesized a series of thiophene trifluoropyruvic acid (HTTA), terephthalic acid (TPA) and o-phenanthroline (Phen) complexes by thermal reflux method. Eu(TPA)(TTA)Phen, Eu2(TPA)(TTA)4Phen2 and Eu(TPA)(TTA)Phen rare earth organic complexes were characterized by elemental analysis, infrared spectroscopy, scanning electron microscope and thermal stability analysis. The emission spectrum wavelength range of Eu (III) complex is 560∼710 nm. The thermal stability order of Eu (III) complexes is: the mononuclear complex Eu(TTA)3Phen< the binuclear complex Eu2(TPA)(TTA)4Phen2< the chain polynuclear complex Eu(TPA)(TTA)Phen. And the formation of the binuclear/polynuclear structure of the new complexes appears to be responsible for the enhancement of their thermal and optical stability. The thermal and optical stability are key factors in the future application of agricultural film. Li et al. [10] synthesized an Eu3+-organic complex with Eu3+ as the luminescent center and TOPO and TTA as ligands (TTA is α-Thiophenyl trifluoroacetone, TOPO is tri-n-octyl phosphine oxide). It is an agricultural light conversion agent that can convert ultraviolet light into red light. The light conversion film prepared by uniformly dispersing the light conversion agent in the resin has been proven to have significant yield increase effects, as shown in Table 4. Xiang et al. [32] developed a new compound β-diketone 4-neneneba methoxy benzoyl tri fluoroacetone (MBPTFA). The Eu (MBPTFA)3Phen ternary complex red light conversion agent was prepared by using MBPTFA and phenanthroline(Phen) as ligands and Eu3+as luminescent centers. Its organic ligand molecules can effectively transfer the absorbed excitation light energy to the central rare earth Eu3+ ion, strongly sensitizing Eu3+ luminescence.Table 4 The comparison of cucumber harvest between light energy conversion film and contrast film [10].

Table 4Type of greenhouse film	Cultivated area (m2)	Yield (kg·m−2)	Equivalent Yield (kg·666.7m−2)	Yield increase (%)	
contrast 1	274	7.3	4869	–	
light-conversion film 1	288	8.6	5729	17.7	
contrast 2	180	6.8	4535	–	
light-conversion film 2	180	7.7	5136	13.3	

The basic research on rare earth organic complexes as light conversion agents mainly focuses on binary and ternary complexes with rare earth ions Eu3+as the luminescent center. β-diketone organic ligands have high light absorption efficiency and can effectively transfer energy to rare earth ions Eu3+, therefore Eu3+- β-Diketone complexes have high luminescence efficiency and are currently one of the most studied rare earth organic complexes [33,34]. The rare earth organic complexes are widely used in the processing and preparation of light conversion agricultural films due to their high compatibility with plastics and simple film preparation process. However, the light conversion agent has problems such as high cost, fast luminescence attenuation, short fluorescence lifetime, strong pollution in the preparation process, and poor matching between characteristic emission spectra and photosynthesis absorption spectra, which limit its application in agricultural light conversion films.

2.1.2 Organic fluorescent dye light conversion agent

Organic fluorescent dye light conversion agent is composed of one or more organic fluorescent substances, which are mainly composed of large organic conjugated system (benzene ring or heterocycle) and its own chromophore and chromophore with electron donating characteristics [26]. It is generally achieved by effectively absorbing ultraviolet light from sunlight using its own chromogenic groups, thereby causing the energy level transitions of π electrons within organic fluorescent molecules to produce corresponding characteristic fluorescence. Organic fluorescent dye conversion agents have good compatibility with resins. By fully mixing them with resins in a certain proportion and blowing, it is easier to obtain uniformly dispersed agricultural conversion films. Kyung et al. [35] synthesized four 3- and 4-position naphthalimide blue fluorescent dyes with diacetylene bonds and prepared them into light conversion agricultural film.And the effects of diacetylene linkage on the fluorescence properties of naphthalimide-based dyes are investigated. The study results showed that the four synthetic fluorescent dyes effectively enhanced the photosynthesis of blue light. and showed high quantum yields ranged in 0.77 and 0.83 where dye 1 emitted at highest quantum yield. The transmittance of the dye-coated PE film using dye 1 at 400–500 nm was observed by 46.8 % higher than that of the transparent film. By using PE film coated by dye 1, fresh weight, dry weight and leaf area of lettuce were significantly increased after three weeks of cultivation. Wang et al. [11] designed and synthesized six triaryl acrylonitrile compounds and one diaryl acrylonitrile compound, and prepared light conversion agricultural films doped with these compounds. After one month of outdoor radiation (summer), their fluorescence intensity decreased to 17–40 % of the basic intensity. Most organic fluorescent dye conversion agents have problems such as high production costs, poor photostability, difficult degradation process, easy oxidation and decomposition, short service life, low absorption and conversion efficiency of sunlight, and easy pollution of the ecological environment, which are not conducive to the development of ecological agriculture [1]. Therefore, there are certain limitations in agricultural applications [9].

2.2 Inorganic light conversion agent

The rare earth inorganic light conversion agents are prepared by directly doping one or more rare earth ions as activators into an inorganic salt matrix. Compared to organic light conversion agents, they have unique advantages such as cheap substrate materials, easy adjustment of light conversion function, stable optical properties, simple production, convenient storage, and low environmental pollution. The unique advantages make them a popular type of agricultural light conversion agent. However, existing inorganic light conversion agents typically have polar, hydrophilic, and high free energy surfaces, and are incompatible with some non-polar, hydrophobic, and low free energy polymers, resulting in poor dispersion and aggregation in agricultural plastic films, resulting in reduced transparency and uniformity of agricultural films, and poor processing performance [24]. In recent years, the surface modification of rare earth inorganic light conversion materials has become an important technological issue that must be solved in the agricultural field of rare earth luminescent materials, in response to the poor compatibility between rare earth inorganic light conversion materials and polymer matrices. Generally, the core/shell modified fluorescent powder with surface coating is prepared by coating one or more layers of inert materials on the surface of fluorescent powder particles [[36], [37], [38], [39], [40]].

Zhu et al. [36] prepared SrAl2O4: Eu2+, Dy3+materials through dry media reaction, and combined them with a light conversion agent through YSiX3 to prepare a composite SrAl2O4: Eu2+, Dy3+ light-emitting material. The SEM results indicate that during the preparation of composite luminescent materials, the light conversion agent is completely coated on the surface of SrAl2O4: Eu2+, Dy3+, and the coated light conversion agent layer is very dense and almost devoid of pores. Under the excitation of ultraviolet light, the emission spectrum shows broadband emission of 450 nm–650 nm, but the color is mainly concentrated in the orange red area that is conducive to crop growth and development, as shown in Fig. 5(a) and (b).Fig. 5 CIE 1931 chromaticity diagram for SrAl2O4:Eu2+,Dy3+and SrAl2O4:Eu2+,Dy3+/light conversion agent (a) and luminescent photograph of SrAl2O4:Eu2+,Dy3+/light conversion agent (b) [36].

Fig. 5

Wang et al. [37] prepared surface active agent modified CaCO3: Eu3+phosphors using carbonization method, and prepared PE films using modified CaCO3: Eu3+phosphors. The experimental results show that compared with oleic acid, stearyl phosphoric acid and Sodium dodecyl sulfate, sodium oleate is the best modifier for CaCO3: Eu3+ phosphor. The CaCO3: Eu3+ phosphor modified by sodium oleate is evenly dispersed in the PE film. The modified PE film can emit uniform red light under the excitation of ultraviolet light, and has stronger absorption of ultraviolet light in the sunlight, which is conducive to the growth of plants. Shi et al. [38] used oxalate as precipitant to precipitate rare earth ions to mix them evenly, and then synthesized boron Yttrium orthovanadate gadolinium europium rare earth inorganic light conversion agent red powder by high-temperature solid state method. Compared with traditional (Y, Gd) VO4: Eu light conversion agent, adding boron with the molar amount of vanadium will not affect the inherent tetragonal crystal system structure of vanadate crystal. The particle size of the light conversion agent is small, easy to crush, and the luminous intensity is increased by more than 10 %, while the reaction temperature is reduced by 100 °C.

In addition, the doping of rare earth europium ions with alkaline earth metal sulfides is also a hot topic direction for light conversion agent materials. Ding et al. [39] developed a series of CaS: Eu2+light conversion materials using solid-state sintering and microwave radiation, and focused on studying and analyzing the optical properties and particle size distribution of the light conversion materials. The results showed that the CaS: Eu2+light conversion agent prepared by microwave radiation method had more uniform particle size distribution and greater luminous intensity. After the silica was coated by coupling agent modification, the compatibility and weather resistance of CaS: Eu2+ light conversion agent with agricultural film were further enhanced. Lian et al. [40] synthesized a core-shell structure CaS: Eu2+@ CaZnOS: Mn2+composite light conversion agent through a two-step solid-state method, as shown in Fig. 6 (a). This light conversion agent has dual UV/green excitation and dual red emission functions, as shown in Fig. 6 (b). The oxygen sulfide coating with thermal and chemical inertness in the core shell structure can prevent sulfide from hydrophilicity, air sensitivity and thermally induced decomposition, which solves the problem of poor hydrolysis stability of sulfide light converter. Therefore, CaS: Eu2+@ CaZnOS: Mn2+light converter can be added to PE film to prepare light conversion agricultural film with long life and strong red light emission.Fig. 6 Schematic diagram of the formation of core-shell structured CaS:Eu2+@CaZnOS:Mn2+ (a) and Comparison of the integrate PLE and PL spectra of CaS:Eu, CaS:Eu@CaZnOS, and CaS:Eu@CaZnOS:0.5%Mn (b) [40].

Fig. 6

Lian et al. [41] synthesized CaS: Eu2+, CaBr2, and CaF2 composite light conversion agents with high luminescence intensity and chemical stability through high-temperature solid-state method. After 180 days of moisture treatment, the double halide modified light conversion agent remained stable and maintained more than half of its initial emission intensity. The composite light conversion agent was immersed in water and still emitted light 60 days later, as shown in Fig. 7(a)–(d).Fig. 7 Powder phosphors of CaS:Eu2+ (A), CaS:Eu2+,CaF2 (B), CaS:Eu2+,CaBr2 (C), and CaS:Eu2+,CaBr2,CaF2 (D).

(a) Comparison of thebody colors without (A–D) and with (A′− D′) moisture treatment for 180 days; (b) Photographs under daylight and UV-lamp excitation (the insets) after immersion for 10 min, 15 days, 30 days, and 60 days; (c) Variations of the emission intensities over time upon excitation by 525 nm; (d) PLE spectra (λem = 650 nm) [41]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7

2.3 Synthesis method of light conversion agent

2.3.1 High-temperature solid-state method

The high-temperature solid-phase method is to weigh the raw materials in a certain chemical ratio, mix them thoroughly and evenly, and then put them into a crucible. They are sintered under high-temperature atmosphere conditions, and then taken out for cooling, crushing, and screening to obtain a solid sample. The high-temperature solid-state method is one of the traditional synthesis methods for synthesizing various luminescent materials [[40], [41], [42], [43]]. The luminescent materials prepared by the high-temperature solid-state method have advantages such as fewer surface defects, excellent crystal structure, long afterglow time, and high luminescence brightness. Moreover, the high-temperature solid-state method is conducive to achieving industrial scale production and application. However, when the calcination temperature of the high-temperature solid phase method is too high and the holding time is too long, it will lead to the sintering and aggregation of particles, uneven particle size distribution, and affect the luminescent performance of the material [12]. In addition, this preparation method usually requires the crushing and screening process to reduce the particle size, which will damage the crystal of the luminescent material, leading to the decline of the luminescent performance of the material. He et al. [44] synthetized a blue-light-emitting carbon dots (CDs) by high-temperature solid-phase method. And blue-light-emitting carbon dots (CDs) were composited with red-light-emitting europium ions (Eu3+) solutions under the synergistic reaction of polyvinyl alcohol (PVA) to prepare the light conversion film. Furthermore, tunable photoluminescence property can be successfully achieved by controlling the ratio of CDs to doped Eu3+, this property can meet the variable light component requirements for different species of plants.

2.3.2 Hydrothermal synthesis

In 1990, Kutty et al. [[45], [46], [47]] reported for the first time that rare earth doped boron Aluminate light converter was successfully synthesized by hydrothermal method. Li et al. [48] synthesized MMoO4 (M = Sr, Ba) nanophosphors doped with europiumvia a facile hydrothermal method using isopropanol. The relationship between phosphor crystalline phase, morphology, photoluminescent properties and hexadecyl trimethyl ammonium bromide (CTAB) concentration, pH value in precursor solution was investigated. The results indicated that the morphology and photoluminescent properties were strongly influenced by CTAB concentration and pH value in precursor solution. In SrMoO4:Eu3+ hosts, the phosphor surface tended to become smoother as the concentration of CTAB was increased; while particles tended to agglomerate as increasing pH value. The Hydrothermal synthesis method uses water as the solvent to heat and pressurize the reaction system in a high-pressure reactor, and then dissolves and recrystallizes insoluble substances in a closed state. After the reaction, the final required luminescent materials are obtained through cooling, drying and other steps. The hydrothermal synthesis of rare earth luminescent materials has the advantages of mild reaction conditions, complete crystal growth, good crystallinity, small particle size, and uniform distribution of the synthesized luminescent materials [10]. However, hydrothermal method belongs to high-pressure synthesis, which requires high structural performance of reaction equipment and is difficult to control the reaction process. Therefore, it has not yet been applied to industrial mass production.

2.3.3 Combustion synthesis

The combustion method can quickly synthesize the required luminescent materials at lower reaction temperatures, making the production process more convenient and having significant energy-saving effects. Shokouhimehr et al. [49] synthesized YVO4:Eu3+ phosphors by simple combustion method using aniline, hydrazine, triethylamine and urea as fuels. The crystallite size of as synthesized YVO4:Eu3+ phosphor by aniline is the smallest, at 22.9 nm. The best photoluminescence properties were achieved when aniline was used as a fuel.

The luminescent material prepared by combustion method has a small particle size, a large specific surface area, and a small decrease in luminescent brightness after grinding the particles [[50], [51], [52]]. Compared with high-temperature solid-phase method, the biggest advantage of combustion method is its high speed and energy saving. However, the initial products synthesized using this method generally have a larger specific surface area and lower density, which will have a certain degree of negative impact on the luminous intensity of the luminescent material.

2.3.4 Sol-gel method

The sol-gel method is a soft chemical method, compounds containing highly chemically active ingredients are uniformly mixed in solution to form precursors, and then transparent and stable sol system is formed through hydrolysis and condensation reaction. The sol is converted into gel. Finally, the required materials are prepared by drying and high-temperature sintering. The reaction temperature of the sol gel method is low, and the reaction is completely carried out in the solution state. It is easy to adjust the concentration ratio of each reaction component, and its dispersion degree can reach the molecular level, so the particle size distribution of the synthesized product is uniform. At present, the sol gel method is highly active in the development of rare earth light-emitting materials synthesis. Many domestic researchers have synthesized rare earth doped light converters by this method and conducted in-depth research on their light-emitting properties [[53], [54], [55], [56], [57]]. Jiang et al. [53] synthesized YAG: Ce3+, Tb3+rare earth phosphors by the sol-gel method. The strongest excitation peak wavelength is 273 nm, and the strongest emission peak wavelength is 545 nm. The phase composition of YAG: Ce3+, Tb3+ rare earth fluorescent powders with a particle size of about 1um is simple. YAG: Ce3+, Tb3+ have uniform powder and uniform luminescence. However, the cost of raw materials used in the sol gel method is high, the reaction operation is complex, the cycle is long, and the shrinkage of the product is high, so it is only limited to the laboratory scale and has not been successfully applied to industrial production.

2.3.5 Microwave radiation method

In recent years, microwave radiation method has a relatively deep and extensive application prospect in the synthesis of rare earth luminescent materials [[58], [59], [60], [61]]. The emergence of microwave radiation method provides an important technological means for the rapid preparation of high-quality new materials. Zhang et al. [59] rapidly synthesized sub-ultrafine CaS: Ce3+, Sb3+ green phosphors using microwave radiation method. The excitation peak and emission peak of the double doped phosphor which was synthesized under the action of microwave field remain constant at 789 nm and 489 nm, respectively. Compared to traditional high-temperature solid-phase synthesis methods, there is no red shift phenomenon with increasing doping ion concentration.The rare earth luminescent materials synthesized by microwave radiation method have advantages such as high doping concentration of rare earth elements, pure phase, and high luminescence intensity. However, the reaction mechanism of this method is still unclear, and further in-depth research is needed on how to effectively control the reaction temperature.

2.3.6 Coprecipitation method

The process of co-precipitation method first involves adding a precipitant to a mixed solution system containing metal cations for co precipitation reaction. Then, the required products are obtained through process steps such as washing, drying, and high-temperature calcination. In recent years, the co-precipitation method has gradually achieved certain research results in the synthesis of rare earth light conversion materials [[62], [63], [64]], and the light conversion materials have excellent and stable luminescent properties. Lai et al. [63] synthesized Y(P,V)O4:Tm3+ phosphors by the co-precipitation method. The particle morphology of Y(P,V)O4:Tm3+phosphor is excellent. Under the excitation of 147 nm vacuum ultraviolet light and 254 nm ultraviolet light, the main emission peak of the phosphor is located at 476 nm.

The coprecipitation method [12] has the advantages of simple and economical process operation, uniform mixture of reactants, low calcination temperature, short reaction time, and good and stable product performance. However, during the preparation process, impurities are easily introduced into the reaction system, resulting in colloidal precipitates that take longer to wash and filter. Therefore, how to choose suitable precipitants and how to effectively control the preparation process conditions have become the key points to solve the problem.

3 The development prospects of transforming light agricultural film

In recent years, the application of light conversion agricultural film in the fields of greenhouse and greenhouse farmland planting has gradually increased. It can not only retain the necessary light source for crop growth and development, but also further adjust the quality of sunlight shining on crops, converting the yellow green and ultraviolet light that is not beneficial or harmful to crop growth and development into blue purple and red orange light that is conducive to crop growth, thereby achieving the goal of increasing production and improving quality. Although China started relatively late in the research of agricultural light conversion film technology, it has developed rapidly and achieved significant results in the promotion and application of agricultural greenhouse in the field.

From the current research results and agricultural application market of light conversion agricultural films, the key to the technological development of light conversion films still lies in the development of light conversion agents, which still need further improvement in the following aspects.(1) Improving the lifespan, weather resistance, and reducing production costs of light conversion agricultural films

Under long-term continuous sunlight exposure, the surface color of rare earth light conversion agents gradually darkens, the luminous intensity gradually decreases, and the transmittance of light conversion agricultural films will decrease to a certain extent, thereby affecting the relevant performance indicators of light conversion agricultural films [8,10]. At present, the effective period of the light conversion function of rare earth light conversion agricultural films developed in China is about 4–6 months [65]. At the same time, the price of rare earth europium compounds in the light-emitting center is relatively expensive, which leads to the high production cost of light conversion agents containing rare earth europium compounds, and to a certain extent, hinders the commercialization of light conversion agricultural films.(2) Strengthen the development of blue light conversion agricultural film and prepare light conversion agricultural film with dual light conversion function

Based on the reported rare earth blue light agricultural films, most studies have directly added organic fluorescent compounds to the resin to make them. The organic fluorescent dye contained in the light conversion agricultural film is easy to decompose under the long-term continuous irradiation of ultraviolet light, causing the molecular structure to be damaged. Therefore, it is of great research significance to research and develop rare earth inorganic blue light conversion agent with excellent performance and stable structure [12,13]. The main research direction in the future is to prepare dual functional light conversion agricultural films with simultaneous red and blue light conversion by adding light conversion agents.(3) Improving the compatibility and dispersibility of light conversion agents in resins

At present, most of the new rare earth light conversion agricultural film products independently developed in China are prepared by directly doping rare earth light conversion agent powder into organic resin. The dispersion and compatibility between the rare earth light conversion agent powder and organic resin still need further improvement. The technical measures to improve the compatibility between light conversion agents and resins include using organic surface modification processes such as sodium oleate and organic silane to treat the light conversion agents, or preparing light conversion agents with small and uniform particle sizes [12,15]. For example, the conventional aqueous precipitation method is changed to the sol gel method or the hydrothermal method to enhance the dispersion and uniformity of the light conversion agent powder in the resin; Add the light conversion agent powder in the form of masterbatch; Make the rare earth organic complex and polyolefin produce strong chemical bond to synthesize "bonded rare earth polymer” [66].(4) The development of weathering resistant and drip free agricultural film through multilayer co-extrusion blow molding technology

The light conversion agricultural film processed by single-layer blow molding technology usually only has insulation and light conversion functions, lacking anti fog function. Therefore, the use of the light conversion agricultural film will cause significant fog in the greenhouse, reducing the yield increase effect of the light conversion agricultural film [12]. If light stabilizers, antioxidants, anti fogging agents, and light conversion agents are layered and doped into the resin, and processed using three-layer co-extrusion blow molding technology to produce weather resistant light conversion drip free agricultural films, the functions of different structural levels can be fully reflected and their light conversion durability can be maintained [15]. It is worth noting that in the process of making agricultural film products, efforts should be made to avoid possible interference between different additives [10]. For example, anti fogging agents containing hydroxyl groups in the molecular structure should be avoided as far as possible to prevent fluorescent quenching of light conversion agents.(5) Developing light conversion agricultural films that meet the growth needs of most plants

Through analyzing and studying the characteristic absorption spectrum and emission spectrum of plant photosynthesis from multiple perspectives such as photoecology, plant physiology and molecular dynamics, we have developed different specifications and types of light conversion agricultural films that can significantly promote the growth of most plants. To improve the growth environment of crops and develop high-tech ecological agriculture by adjusting the quality of sunlight [6,12].

In order to solve the current problems, the focus should be on utilizing the advantages of existing light conversion agents and combining them with other materials or fluorescent carriers with excellent fluorescence characteristics (For example, inorganic light conversion agents and organic light conversion agents develop towards organic-inorganic composite light conversion agents) to prepare the new light conversion agents with dual light conversion function, stronger red light emission levels, and better stability. Transforming light is a new agricultural technology and concept, which is currently a major direction of agricultural development. The promotion and application of light conversion agricultural film is in line with the national "three rural” policies, and is conducive to the development of photon ecological pollution-free planting technology. It will lead a new revolution in agricultural technology and agricultural film products. Therefore, it is particularly important to raise the level of attention paid to the research of light conversion films beyond “the surface of paper” [[67], [68], [69]].

Due to the unique functions and wide applicability of light conversion films, it is necessary to promote their technological progress and production development through a combination of government, industry, academia, and research [70,71].

Ethics and consent statement

1. Review and/or approval by an ethics committee was not needed for this study because this is a review article about rare earth agricultural light conversion film and does not involve the research section that requires ethical approval;

2. All participants provided written informed consent to participate in the study and for their data to be published;

3. We didn't study live vertebrates and higher invertebrates in our work;

4. Our research didn't involve the interaction with or the observation of people, and/or the use of peoples' data;

5. Our work didn't have the potential to pose a biosecurity threat.

Data availability statement

All the relevant data are included in the manuscript. No separate repository is attached.

CRediT authorship contribution statement

Jing Wang: Writing – review & editing, Writing – original draft. Xin Qiao: Conceptualization. Bo Li: Validation. Bo Liu: Formal analysis. Juan Zhang: Validation. Zhen Yan: Methodology. Pengcheng Hao: Supervision. Xuanhang Wang: Formal analysis. Yayuan Liu: Formal analysis. Leijun Shen: Writing – review & editing. Zhongzhi Wang: Validation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This research received no external financial or non-financial support.

There are no additional relationships to disclose.

There are no patents to disclose.

There are no additional activities to disclose.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Thank you very much for the strong support of Baotou Research Institute of Rare earths. Thank you very much to the reviewers and editors for their constructive comments.
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